This invention relates to a battery separator for controlling copper ion diffusion in alkaline rechargeable batteries.
A zinc based battery presents many challenges for the battery manufacturer. One of these challenges involves containing zincate diffusion within the battery. This is a particularly severe problem because zinc is extremely soluble in the strongly alkaline environments routinely used as the electrolyte in batteries. The presence of KZn(OH)x where x=1 to 3 presents a double-edged sword for the battery designer. The high solubility allows for rapid current spikes typically unattainable with other battery systems. On the other hand, this high solubility diffuses zinc ions in undesired locations within the battery, which upon re-plating leads to the well-known phenomenon of electrode shape change within zinc batteries. This shape change entails an agglommeration of the zinc towards the center of the battery with concomitant depletion from the edges.
Additionally, and more seriously, a problem arises from this high zinc mobility such that there is an accumulation of zinc dendrites within the separator as the battery cycles. Soluble zinc, driven by the electric fields of the battery, finds its way through the pores of the separator. A concentration gradient within the separator leads, upon zinc re-plating, to dendrite formation. The dendrites are small tree-like metal formations that are capable of ripping the separator and prompting a cell short.
One approach presented disclosed in concurrently filed co-pending application Ser. No. 09/839,324, entitled Homogenous Seperator, the disclosure of which is incorporated herein by reference, is to use separators impregnated with copper salts. These separators contribute to a substantial diminution of zinc migration from the anode to the cathode. This behavior is a result of the copper salts complexing the cellulose and thus preventing access of zinc to the pores of the cellulose. A problem with this approach is that copper tends to leach out from the separator at rates that affect battery performance. Leached-out copper tends to plate on zinc and modify the overvoltage potential. It also leaches out and re-plates on adjacent separators, affecting their ion transport properties.
Prior art in this area has focused on doping porous polyolefins with inorganic fillers. Thus, Machi et al in U.S. Pat. No. 4,273,840 disclose a multi-layer battery separator, wherein a metal ion binding substance is incorporated into a layer of synthetic resin that is sandwiched between two layers of synthetic resin. The first layer and third layers contain a monomer having a grafted hydrophilic group, and the middle layer with the metal ion-binding substance is fusion-bonded to the first and third layers. When the separator is to be used in zinc-silver oxide batteries, inorganic sulfides such as zinc sulfide, magnesium sulfide and calcium sulfide are used. All layers to be fused are preferably composed of polyethylene, polypropylene and other olefinic thermoplastic resins.
Ondeck et al in U.S. Pat. No. 5,948,557 disclose a battery separator made of a microporous material comprised of a linear polyolefin containing finely divided, substantially water-insoluble filler particles, such as zinc sulfide, distributed throughout the matrix. Microporous filaments and fibers are made wettable in U.S. Pat. No. 5,126,219 by incorporating hygroscopic fillers into the composition.
A sulfide additive is added to the alkaline electrolyte in U.S. Pat. No. 4,078,127. The purpose is to stabilize the divalent silver oxide (AgO) in the cathode.
The present invention provides a separator that diminishes the availability of soluble copper ions emanating from a zinc-resistant copper-doped, regenerated cellulose separator.
In accordance with the invention, a cellulose-based separator is embedded with an insoluble sulfide salt in a concentration range of 2 to 25% weight of said separator. The separator of the invention, when placed next to a copper-containing separator, minimizes copper ion diffusion into the electrodes by precipitation of said ions. The present invention relies on the dispersion of a sufficient amount of sulfide salt in the regenerated cellulose film as to substantially decrease the concentration of copper ions in the battery.